Imagers are useful in recording features of an electromagnetic field, e.g. light, at a detector location and converting the recorded features into an image that can be stored or displayed for various purposes. In the example of every day cameras, imagers capture visible light incident upon a sensitive film or digital array. In analog film systems, the film is altered so as to capture a snapshot in time of the light to which it was exposed. In digital imaging systems, an array of light-sensitive pixels respond to an intensity of incident light at each pixel location, providing an electrical output representative of the incident light. The output of an imager can be referred to as an image.
Motion or video cameras repeat the process described above, but permit a time-sequence to be captured, for example at regular intervals, so that the captured images can be replayed to recreate a dynamic scene or sequence.
Most film and digital pixel imagers include wavelength-specific sensors or detectors. The chemical composition of the film or the design of the digital pixels and associated filters determines the range of wavelengths of light to which the film or pixels respond. Practically, a detector or imager has a frequency response that is optimized to provide images of light in the range of wavelengths the imager is designed for. The most common examples are sensitive to visible light (e.g., red, green, blue, and combinations thereof). Visible light corresponds to the range of wavelengths of electromagnetic radiation to which our eyes are sensitive, and is generally in the range of 400 to 750 nanometers (nm).
Special film and digital pixel imagers are designed for low-light operation to provide night vision capability for military, security, or other special applications in which an illumination source is not available to cause a visible light image. Low-light or night vision imagers rely on detecting and imaging frequencies below (wavelengths longer than) the visible (red) wavelengths, and are sometimes called infra-red (IR) detectors. IR detection is more suited for picking up heat emissions from objects such as a person's body or a vehicle. IR radiation itself can be roughly divided into sub-spectra including the near-infra-red (NIR) having wavelengths between about 750 to 1100 nm, short-wave-infra-red (SWIR) having wavelengths between about 1100 and 2500 nm, medium-wave-infra-red (MWIR) having wavelengths between about 2500 and 8000 nm, and long-wave-infra-red (LWIR) having wavelengths between about 8000 and 12000 nm. These ranges are defined somewhat arbitrarily, and are given merely for simplifying the following discussion, and those skilled in the art will appreciate the generality of the discussion as it relates to the bands of wavelengths of the electromagnetic spectrum.
Present visible light imaging cameras have used silicon devices made with CID, CCD, or CMOS APS architectures. The low cost and efficient collection of photons from 400-750 nm wavelengths has enabled silicon devices. Extending the use of silicon imagers into the near infrared (NIR) band requires a greater volume of material to detect these wavelengths because of silicon's relatively low absorption coefficient in this wavelength range. This increases the size of the detectors and causes increased leakage current and requires expensive manufacturing processes or higher voltages to operate. The use of thick silicon substrates also limits the ability to integrate other devices, and impedes the ability to place circuits under the detector.
SWIR remains mostly undetected by silicon (Si) detectors, which are transparent to this wavelength. The use of exotic detector materials such as indium gallium arsenide (InGaAs) or germanium (Ge) can solve some technical problems but creates others and increases the cost of the products.
With LWIR, direct detection is typically attempted with exotic materials such as HgCdTe cooled to cryogenic temperatures. This is not ideal as such cooled exotic detectors do not lend themselves for practical use in many applications.
A bolometer is one way to detect electromagnetic radiation such as IR. A suspended or cantilevered member, e.g. in a micro electromechanical system (MEMS), responds to detected radiation by way of changes in its resistance or physical dimension to indicate an intensity of the incident radiation. Bolometers can thus be used in arrays as pixels in an imaging system. The deficiencies outlined above with regard to detection or IR and broadband light in traditional detector materials applies to bolometer designs, and thus even with bolometer detectors, present systems are inefficient and impractical over broad ranges of the IR spectrum, notably in or around the IR wavelengths. A microbolometer is a specific type of bolometer. Infrared radiation strikes the detector material, heating it, and thus changing its electrical resistance as mentioned above. This resistance change is measured and can be used to create an image. A microbolometer can absorb and detect MWIR as well as LWIR radiation. Unlike other types of infrared detecting equipment, microbolometers do not require cooling.
Present low-light or night vision IR imagers are usually less sensitive than would be desired, lack color definition, and have limited frequency response. Also, low-light imagers can be more costly, noisy, and require greater circuit resources than visible light imagers to achieve useful gains in low-signal conditions. Furthermore, because long wavelength (IR) light penetrates common detector materials to a greater depth, IR detectors are larger than would be desired for compact portable applications. In summary, present IR detectors are not economical and do not provide good quality imaging, especially over a range of wavelengths that might be desired.
The reading out of information retrieved by imaging array pixels requires significant bussing and addressing circuits. These busses and circuits require additional layers fabricated above the base substrate layers which are located around the optical path of the pixel and can block the incident light along certain paths. As such, the busses and circuits simultaneously reduce the amount of area that the pixel can dedicate to photon collection (sometimes referred to as the pixel fill factor) and generally create a vertical light tunnel for incident light to reach a given pixel. Reducing the pixel fill factor reduces the sensitivity of a device.
An imaging circuit can be characterized by a “device fill factor,” corresponding to the fraction of the overall chip area being effectively devoted to the pixel array, and a “pixel fill factor,” corresponding to the effective area of a light sensitive photodiode relative to the area of the pixel that may be used to determine the amount of silicon that is photoactive. The device fill factor in conventional devices is less than unity (1.0) because a notable portion of the device beneath the pixel array area cannot be used for processing.
Moreover, the pixel fill factor in conventional devices is typically substantially less than 1.0 because, for example, bussing and addressing circuits are fabricated around the base substrate layers of a pixel. As such, the bussing and addressing circuits limit the amount of space available for photodetection circuitry. Such bussing and addressing circuitry also limit the acceptance cone angle for electrons directed towards an imaging array.
An exemplary conventional CMOS imaging circuit commonly used in the industry, the MT9T001 CMOS Digital Image Sensor from Micron Technology, Inc., has a pixel fill factor of approximately 28% and a device fill factor of approximately 57%. As such, approximately 0.28 times 0.57, i.e. 16% of the semiconductor material of a conventional CMOS imaging circuit is photoactive. In other words, approximately 84% of a CMOS imaging circuit is used for purposes other than the primary purpose of the circuit, which is photodetection. This inefficiency leads to unwanted increased size of the overall product and cost of the product as well as degraded performance of the product made from the conventional photodetector array. An improved photodetector and array is needed that overcomes some or all of the above-mentioned disadvantages.